There are fewer flights/itineraries.
Chinese authorities have imposed a strict policy regarding international flights with quotas and circuit-breakers (“Five Ones Policy”, article on this from Chinese Wikipedia). The number of seats is also limited to 75% of ordinary capacity.
Under “Five Ones Policy”, for most international routes, one airline can only operate one route from one country to Mainland China with one flight per one week.
Circuit-breakers also mean all flights from the airline (on that route) will be cancelled for two or four weeks if the number of CoVID-positive passengers on arrival reach certain numbers. Passengers booked on cancelled flights of course will have priority on subsequent resumed flights, causing overbooking (or causing companies to restrict number of available fares that can be sold in advance).
This policy is also subject to countermeasures from e.g. France and Canada (article in Chinese), because Chinese airlines gain an unfair advantage due to the number of Chinese airlines (since the restriction is nominally per airline). USDOT’s ban against Chinese-operated flights last May/June was also related.
France’s and USA’s countermeasures were largely diplomatically resolved, although the number of flights is still extremely limited. Canada’s dispute is still ongoing and has had a dramatic effect on already high ticket prices.
Not to mention, many Chinese consulates are unable to issue the necessary boarding health declaration code to many transit passengers as the required tests must be done in the country of departure of the leg to China. Some consulates impose residency requirements and refuse to issue the code to residents of a country where a direct flight is available, e.g.
Therefore Starting from November 6, the Chinese Embassy/Consulates in Germany will in principle only examine and verify the green health code with the “HS” mark or the health declaration of the passengers bound for China via direct flights from Germany to China.
Additionally, the demand is particularly higher now since many expats have not been able to visit home during the past year and summer vacation is coming up for international students.